Which of these numbers is composite? ${2,\ 11,\ 31,\ 51,\ 59}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 2, 11, 31, and 59 each have only two factors. The factors of 51 are 1, 3, 17, and 51. Thus, 51 is the composite number.